Agreed: the usual velocity 'claim' is a 10% increase with the imp. ctg.

The original .250 Savage was SAAMI loaded to 53,000 psi. The .257 Roberts SAAMI stanard was 54,000psi.
In a good bolt actioned rifle, there is no reason this must be adhered to.
This case is the parent of the .22-250, which has a SAAMI loaded pressure limit of 65,000psi.
Thus, in both the .250 IMP and .257 IMP, working pressures are usually higher than the original ctg. was loaded to & in part due to that, the velocities are higher.